1. Field of the Invention
This invention relates to an improved front projection display screen which efficiently spreads over a viewing space the image light from a projector while absorbing background light from other sources both inside and outside the viewing space, thus providing an image of improved contrast under practical conditions of ambient light and projector power.
2. Description of the Prior Art
Earliest front projection screens were simply matte white surfaces such as a white sheet or a coat of dull white paint. Matte surfaces have two flaws which make them useful only in dark viewing areas or with very powerful projectors. First, they use image light inefficiently by spreading it into much of the half space in front of the screen, so that most goes in directions in which there are no viewers. Second, matte surfaces efficiently return to the viewing space much of the light striking them from sources of background light. It is well known that such background light washes out image contrast and color.
To improve front projection screens there have been proposed various forms which comprise a multiplicity of optical cells each of a size smaller than can be resolved by the eyes of a viewer. The proposed improvements lie in the internal structure of the cells and it is convenient to describe just the cell structure it being understood that the optical cells are meant to be arranged side by side to form a screen.
Prior art optical cells are of two kinds. The first kind of cell simply concentrates image light into a solid angle which is smaller than that illuminated by a matte surface screen. Thus image light within this solid angle is increased relative to the case of a matte surface by a factor called gain. Such screens are called gain screens. Gains on the order of 10 are achievable without much loss in viewing angle, where viewing angle is the angle from the screen normal to the most laterally displaced viewer. Higher gains generally require that the viewing angle be decreased in order to squeeze the viewers inside the solid angle illuminated by screen optical cells.
The optical cells of such simple gain screens contain no light absorbing material. Thus background light striking them must return to the front half space and some of it will be returned to the viewing space. Moreover, much of the background light from within the viewing space will be returned to other parts of the viewing space with the gain of the screen. Unless the whole room is blacked out, sources of background light within the viewing space are inevitable. Besides obvious, active sources such as lamps, there are large area sources such as furniture, walls, and the viewers themselves which are, if visible to the eye, also sources of background light. Thus simple gain screens show poor image contrast in many practical combinations of background lighting and projector power. Nevertheless, all front projection screens known to me employ optical cells of this first kind.
The second kind of optical cell contains light absorbing material and optical elements combined in some way intended to perform all of the following functions: to absorb background light, to let pass image light, and to spread image light back throughout the viewing space. Such discrimination between background light and image light is really directional discrimination, based on the fact that image light strikes each optical cell from the direction of the projector while background light strikes from many other directions. In the second kind of optical cell the optical elements commonly confer screen gain as well as act as part of the discriminating scheme.
Heretofore inventors have attempted to effect discrimination between image light and background light by converging the image light to a focus or a near focus at an aperture or a small mirror in a surrounding blackened surface. Because background light does not come from the direction of the projector it converges at some other place within the cell, thus missing the aperture or small mirror and being absorbed by the blackened surface. This approach may be termed discrimination by focus. However, light turned back from a focus just retraces the paths that focused it, thus returning in a parallel beam toward the projector and not spreading across the viewing space. Heretofore inventors of background discriminating reflection screens have retained the idea of directional discrimination by focus and attempted to avoid the retrace problem. The resulting optical cell designs suffer from complexity, difficulty of manufacture, and sometimes imperfect discrimination. A manufacturable screen for small room viewing must have optical cells of largest face dimension about 1 millimeter and internal structure smaller by at least a factor of 10. It must contain from about 200,000 to about 2,000,000 such cells and all of them must closely match each other in optical performance in order to avoid mottled and uneven images.